1. Field of the Invention:
The present invention is related to a semiconductor element gate circuit and more particularly to a gate circuit of a gate turn-off thyristor.
2. Description of the Prior Art:
High power gate turn-off thyristors (GTO) have been developed in recent years whose voltages and turn-off current are approximately 2500 volts and 1400 amperes respectively. These GTO's are commercially available and are supplied for use with inverters or choppers. The value of the anode current which can be turned off by negative current flowing to the gate of the GTO is expressed by the term "turn-off current" and is identified as I.sub.TGQ. The negative minimum current which flows to the gate for turning off the GTO is identified by I.sub.GQ with the ratio I.sub.TGQ /I.sub.GQ being termed the turn-off gain which has a value of between 3 and 5.
Consequently, a current of approximately 300 amps must flow in the gate in order to turn off a GTO of I.sub.TGQ =1000 A. It is required that the gate trigger current to turn on the GTO has approximately the same order of magnitude as an ordinary thyristor. Once a large gate current flows in the negative direction, the GTO can be reignited even if a slight gate current flows in a positive direction. The gate current is inverted, for example, by the effect of the stray inductives between a pulse transformer and the GTO in the gate circuit utilizing capacitor discharge. Even inserting a high speed diode in the output terminal of a secondary winding of the pulse transformer has no effect in preventing this phenomena, because a current flows corresponding to the amount of the inverse recovery charge.
The gate circuit of FIG. 1 attempts to deal with this phenomena with the circuit being described in U.S. patent application Ser. No. 242,158 filed Mar. 9, 1981. In order to provide a simple explanation of its operation, the FIG. 1 shows a DC power supply 11, a diode 12, a capacitor 13, a pulse transformer 14 and the primary windings 15 and 16 of the pulse transformer 14. A secondary winding 17 of the pulse transformer 14, a transistor 18, a GTO 19 and a diode 20 complete the FIG. 1.
If the voltage of the DC power source 11 is represented by the voltage E and the voltage of the capacitor 13 as E.sub.C then the voltage E.sub.C in the steady state becomes larger than the voltage E.
Assuming that the number of turns of the primary windings 15 and 16 are equal and that the ratio of the number of turns of the primary winding 15 or 16 to that of secondary winding 17 is n.sub.1, then the following observations with regard to the FIG. 1 are in order.
When the transistor 18 is turned on, the voltage E.sub.C is applied to the terminals B-C of the primary windings 16, and the potential of the point A becomes the voltage 2E.sub.C.
Thus, the diode 12 is reverse-biased and no current flows from the DC power source 11. A voltage E.sub.C /n.sub.1 appears on the secondary winding 17. If the resistance of the GTO 19, as seen from the primary winding 15 or 16, is labeled R and the capacitor of the capacitor 13 is C, the capacitor 13 discharges with the time-constant CR and the voltage across the two terminals of the capacitor 13 quickly reach the voltage E/2. The potential of the point A then becomes the voltage E and the capacitor voltage E.sub.C drops slightly. The diode 12 is then forward-biased and becomes conductive. As a result, the voltage E is applied to the series circuit of the primary windings 15 and 16 of the pulse transformer 14, and the voltage of the secondary winding 17 is reduced to E/2n.sub.1.
When the transistor 18 turns off, part of the excitation energy of the pulse transformer 14 goes to the GTO 19, while the remainder of the excitation energy is used for charging the capacitor 13. Most of the excitation energy of the pulse transformer 14 is used to charge the capacitor 13 and consequently the voltage E.sub.C becomes larger than the voltage E.
The gate circuit of FIG. 1 is used in a GTO off-gate circuit, however, with a further increase of the GTO turn-off current I.sub.TGQ, the following problems arise when, for example, I.sub.TGQ =3,000 A:
(i) The stray inductance between the GTO gate and a pulse transformer is ordinarily 1.mu. H, so that the current arise di.sub.RG /dt of the gate turn-off current I.sub.GQ is approximately 30 A/ .mu.s. Assuming that 1,000 A is necessary for gate turn-off current I.sub.GQ, the period T taken for the gate turn-off current to reach 1,000 A will be at least 33 .mu.s. Because, in reality, the decrease of the anode current is even further delayed than exemplified above, an obstacle will have been presented to a 1 KHz switching operation;
(ii) An increase in the voltage of the secondary winding of the pulse transformer causes a rise in the gate turn-off current I.sub.GQ. The voltage of the secondary winding of the pulse transformer 14 is presently approximately 30 to 40 volts. In the instance where the voltage of the secondary winding is 100 volts, di.sub.rG /dt is made 3 times larger than that of the pulse transformer 14. Thus, the period T is reduced to approximately 1/3 of that of the pulse transformer 14 whose voltage of the secondary winding is approximately 30 to 40 volts. However, the reverse breakdown voltage across the gate-cathode of the GTO is approximately 15 volts so that it is undesirable to apply high voltage across the gate cathode in the condition in which there is no GTO anode current. This is so because not only would this high voltage lead to large losses in gate circuit but it might also lead to local heating of the GTO itself. Therefore, simply raising the voltage does not provide for an appropriate solution to this problem.